Acrylate contact adhesive materials having tight molecular weight distribution

ABSTRACT

An initiator system for radical polymerization, which comprises a combination of compounds of the formulae:

This is a 371 of PCT/EP01/08743 filed Jul. 27, 2001 (internationalfiling date).

The invention relates to an initiator system based on nitroxides forfree-radical polymerization of (meth)acrylic acid and/or derivativesthereof and to a process for preparing acrylic pressure sensitiveadhesives (PSAs) with narrow molecular weight distribution using saidinitiator system.

For industrial PSA tape applications it is very common to usepolyacrylate PSAs. Polyacrylates possess a variety of advantages overother elastomers. They are highly stable toward UV light, oxygen, andozone. Synthetic and natural rubber adhesives normally contain doublebonds, which make these adhesives unstable to the aforementionedenvironmental effects. Another advantage of polyacrylates is theirtransparency and their serviceability within a relatively widetemperature range.

Polyacrylate PSAs are generally prepared in solution by free radicalpolymerization. The polyacrylates are generally applied to thecorresponding backing material from solution using a coating bar, andthen dried. In order to increase the cohesion, the polymer iscrosslinked. Curing proceeds thermally or by UV crosslinking or by EBcuring (EB: electron beams). The operation described is relativelycostly and ecologically objectionable, since as a general rule thesolvent is not recycled and the high consumption of organic solventsrepresents a high environmental burden.

Moreover, it is very difficult to produce PSA tapes with a high adhesiveapplication rate without bubbles.

One remedy to these disadvantages is the hotmelt process in thisprocess, the PSA is applied to the backing material from the melt.

However, this new technology has its limitations. Prior to coating, thesolvent is removed from the PSA in a drying extruder. The dryingoperation is associated with a relatively high temperature and shearingeffect, so that high molecular weight polyacrylate PSAs in particularare severely damaged. The acrylic PSA gels, or the low molecular weightfraction is greatly enriched as a result of molecular weight breakdown.Both effects are undesirable, since they are disadvantageous for theapplication. Either the adhesive can no longer be applied, or there arechanges in its technical adhesive properties, since, for example, when ashearing force acts on the adhesive the low molecular weight fractionsact as lubricants and so lead to premature failure of the adhesive.

One solution to mitigating these disadvantages is offered bypolyacrylate adhesives with a low average molecular weight and narrowmolecular weight distribution. In this case the fraction of lowmolecular weight and high molecular weight molecules in the polymer isgreatly reduced by the polymerization process. The reduction in the highmolecular weight fractions reduces the flow viscosity, and the adhesiveshows less of a tendency to gel. As a result of the reduction in the lowmolecular weight fraction, the number of oligomers which reduce theshear strength of the PSA is lessened.

A variety of polymerization methods are suitable for preparing lowmolecular weight PSAs. The state of the art is to use regulators, suchas alcohols or thiols, for example (Makromoleküle, Hans-Georg Elias, 5thEdition, 1990, Hüthig & Wepf Verlag, Basel). These regulators reduce themolecular weight but broaden the molecular weight distribution.

Another controlled polymerization method used is atom transfer radicalpolymerization ATRP, in which initiators used preferably includemonofunctional or difunctional secondary or tertiary halides and, forabstracting the halide(s), complexes of Cu, Ni, Fe, Pd, Pt, Ru, Os, Rh,Co, Ir, Cu, Ag or Au [EP 0 824 111; EP 0 826 698; EP 0 824 110; EP 0 841346; EP 0 850 957]. The various possibilities of ATRP are furtherdescribed in U.S. Pat. No. 5,945,491, U.S. Pat. No. 5,854,364, and U.S.Pat. No. 5,789,487. Generally, metal catalysts are used, which have theside effect of adversely influencing the aging of the PSAs (gelling,transesterification). Moreover, the majority of metal catalysts aretoxic, discolor the adhesive, and can be removed from the polymer onlyby complicated precipitations.

A further variant is the RAFT process (reversible addition-fragmentationchain transfer). The process is described at length in WO 98/01478 andWO 99/31144, but in the manner set out therein is unsuited to thepreparation of PSAs, since the conversions achieved are very low and theaverage molecular weight of the polymers prepared is too low for acrylicPSAs. Accordingly, the polymers described cannot be used as acrylicPSAs.

U.S. Pat. No. 4,581,429 discloses a controlled free-radicalpolymerization process. As its initiator the process employs a compoundof the formula R′R″N—O—X, in which X denotes a free radical specieswhich is able to polymerize unsaturated monomers. In general, however,the reactions have low conversion rates. A particular problem is thepolymerization of acrylates, which takes place only with very low yieldsand molecular weights.

WO 98/13392 describes open-chain alkoxyamine compounds which have asymmetrical substitution pattern.

EP 735 052 A1 discloses a process for preparing thermoplastic polymershaving narrow polydispersities.

WO 96/24620 describes a polymerization process in which very specificradical compounds, such as phosphorus-containing nitroxides, forexample, are described.

WO 98/30601 discloses specific nitroxyls, based on imidazolidine.

WO 98/4408 discloses specific nitroxyls, based on morpholines,piperazinones, and piperazinediones.

DE 199 49 352 A1 discloses heterocyclic alkoxyamines as regulators incontrolled free-radical polymerizations. Corresponding furtherdevelopments of the alkoxyamines or of the corresponding free nitroxidesimproved the efficiency for the preparation of polyacrylates. [Hawker,C. J., paper, National Meeting of the American Chemical Society in SanFrancisco, Spring 1997; Husemann, M., IUPAC World-Polymer Meeting 1998,Gold Coast, Australia, paper on “Novel Approaches to Polymeric Brushesusing ‘Living’ Free Radical Polymerizations” (July 1998)]

In the abovementioned patents and papers attempts were made to improvethe control of free-radical polymerization reactions. There neverthelessexists a need for a nitroxide-controlled polymerization process which ishighly reactive and can be used to realize high conversions incombination with high molecular weight and low polydispersity. Thisapplies in particular to the copolymerization of acrylic PSAs, wherehigh molecular weights are essential for PSA applications.

It is an object of the invention, therefore, to provide an initiatorsystem for a corresponding polymerization process, and to offer apolymerization process, which does not have the disadvantages of theaforementioned prior art, or at least not to so great an extent.

Surprisingly it has been found that asymmetric alkoxyamines of type(II), in conjunction with their free nitroxyl precursors and aslow-thermal-decomposition azo or peroxo initiator, allow polymerizationfor the preparation of acrylic PSAs very effectively and rapidly atrelatively high temperatures.

Claim 1 accordingly provides an initiator system for free-radicalpolymerizations, composed of a combination of compounds of the generalformulae

in which

-   -   R′, R″, R′″, R″″ are chosen independently of one another and are        -   a) branched and unbranched C₁ to C₁₈ alkyl radicals; C₃ to            C₁₈ alkenyl radicals; C₃ to C₁₈ alkynyl radicals;        -   b) C₃ to C₁₈ alkynyl radicals; C₃ to C₁₈ alkenyl radicals;            C₁ to C₁₈ alkyl radicals substituted by at least one OH            group or a halogen atom or a silyl ether;        -   c) C₂-C₁₈ hetero alkyl radicals having at least one oxygen            atom and/or an NR group in the carbon chain; R being chosen            from one of groups a), b), and d) to g),        -   d) C₃-C₁₈ alkynyl radicals, C₃-C₁₈ alkenyl radicals, C₁-C₁₈            alkyl radicals substituted by at least one ester group,            amine group, carbonate group and/or epoxide group and/or by            sulfur and/or by sulfur compounds, especially thioethers or            dithio compounds;        -   e) C₃-C₁₂ cycloalkyl radicals        -   f) C₆-C₁₀ aryl radicals        -   g) hydrogen;    -   X represents a group with at least one carbon atom and is such        that the free radical X• derived from X is able to initiate a        polymerization of ethylenically unsaturated monomers.

Halogens are preferably F, Cl, Br or I, more preferably Cl and Br. Asalkyl, alkenyl and alkyl radicals in the various substituents, bothlinear and branched chains are outstandingly suitable.

Examples of alkyl radicals containing from 1 to 18 carbon atoms aremethyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl,2-pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, t-octyl, nonyl, decyl,undecyl, tridecyl, tetradecyl, hexadecyl and octadecyl.

Examples of alkenyl radicals having from 3 to 18 carbon atoms arepropenyl, 2-butenyl, 3-butenyl, isobutenyl, n-2,4-pentadienyl-,3-methyl-2-butenyl, n-2-octenyl, n-2-dodecenyl, isododecenyl and oleyl.

Examples of alkynyl having from 3 to 18 carbon atoms are propynyl,2-butynyl, 3-butynyl, n-2-octynyl and n-2-octadecynyl.

Examples of hydroxyl-substituted alkyl radicals are hydroxypropyl,hydroxybutyl or hydroxyhexyl.

Examples of halogen-substituted alkyl radicals are dichlorobutyl,monobromobutyl or trichlorohexyl.

An example of a suitable C₂-C₁₈ hetero alkyl radical having at least oneoxygen atom in the carbon chain is —CH₂—CH₂—O—CH₂—CH₃.

Examples of C₃-C₁₂ cycloalkyl radicals include cyclopropyl, cyclopentyl,cyclohexyl or trimethyl-cyclohexyl.

Examples of C6-C10aryl radicals include phenyl, naphthyl, benzyl, orfurther substituted phenyl radicals, such as ethyl, toluene, xylene,mesitylene, isopropylbenzene, dichlorobenzene or bromotoluene.

The above listings serve only as examples of the respective groups ofcompounds, and make no claim to completeness.

In one particularly preferred embodiment of the invention a combinationof the compounds (Ia) and (IIa) is used as initiator system.

In one very advantageous further development of the inventive initiatorsystem, further free-radical initiators for the polymerization arepresent in addition, especially thermally decomposing radical-formingazo or peroxo initiators. In principle, however, all customaryinitiators which are known for acrylates are suitable for this purpose.The production of C-centered radicals is described in Houben Weyl,Methoden der Organischen Chemie, Vol. E 19a, pp. 60-147. These methodsare employed, preferentially, in analogy.

Examples of radical sources are peroxides, hydroperoxides, and azocompounds; some nonlimiting examples of typical radical initiators thatmay be mentioned here include potassium peroxodisulfate, dibenzoylperoxide, cumene hydroperoxide, cyclohexanone peroxide, di-t-butylperoxide, azodiisobutyronitrile, cyclohexylsulfonyl acetyl peroxide,diisopropyl percarbonate, t-butyl peroctoate, benzpinacol. In one verypreferred version, 1,1′-azobis(cyclohexanecarbonitrile) (Vazo 88™ fromDuPont) is used as free-radical initiator.

The compounds of the formula (II) are present preferably in an amount offrom 0.0001 mol % to 1 mol %, more preferably in an amount of from0.0008 to 0.0002 mol %, based on the monomers. The compounds of theformula (I) are present preferably in an amount of from 1 mol % to 10mol %, more preferably in an amount of from 3 to 7 mol %, based oncompound (II). The thermally decomposing initiator from c) is presentwith particular preference in an amount of from 1 to 10 mol %, morepreferably in an amount of from 3 to 7 mol %, based on compound of theformula (II).

For initiation, the cleavage of the X—O bond of the initiator componentof the formula (II) is essential. The cleavage of the bond is broughtabout preferably by ultrasound treatment, heating or exposure toelectromagnetic radiation in the wavelength range of γ radiation, or bymicrowaves. More preferably the cleavage of the C—O bond is broughtabout by heating and takes place at a temperature of between 70 and 160°C.

After the polymerization step is over, the reaction mixture can becooled to a temperature below 60° C., preferably to room temperature.

The invention further provides a process for preparing acrylic pressuresensitive adhesives, in which a monomer mixture composed to the extentof at least 70% by weight of ethylenically unsaturated compounds,especially of (meth)acrylic acid and/or derivatives thereof, issubjected to free-radical polymerization using the inventive initiatorsystem described.

A preferred monomer mixture is one composed of at least 70% by weight ofacrylic monomers of the general formula

where R₁=H or CH₃ and R₂=H or is an alkyl chain having 1-20 carbonatoms.

In one advantageous embodiment of the inventive process vinyl compoundsare used additionally as Monomers, with a fraction of up to 30% byweight, in particular one or more vinyl compounds chosen from thefollowing group: vinyl esters, vinyl halides, vinylidene halides,nitrites of ethylenically unsaturated hydrocarbons.

Examples that may be mentioned here of such vinyl compounds includevinyl acetate, N-vinylformamide, vinylpyridines, acrylamides, acrylicacid, hydroxyethyl acrylate, hydroxyethyl methacrylate, ethyl vinylether, vinyl chloride, vinylidene chloride, acrylonitrile, maleicanhydride and styrene, without wishing to be unnecessarily restricted bythis listing. It is also possible to use all other vinyl compounds whichfall within the group specified above, and also all other vinylcompounds which do not fall within the classes of compounds specifiedabove.

For the polymerization the monomers are chosen such that the resultingpolymers can be used as industrially useful PSAs, especially in such away that the resulting polymers possess pressure-sensitive adhesiveproperties in accordance with the “Handbook of Pressure SensitiveAdhesive Technology” by Donatas Satas (van Nostrand, New York 1989). Forthese applications, the static glass transition temperature of theresulting polymer is advantageously below 25° C.

The polymerization may be conducted in the presence of one or moreorganic solvents and/or in the presence of water. In one advantageousembodiment of the process there are additional cosolvents or surfactantspresent, such as glycols or ammonium salts of fatty acids.

Preferred processes use as little solvent as possible. Suitable organicsolvents or mixtures of solvents are pure alkanes (hexane, heptane,octane, isooctane), aromatic hydrocarbons (benzene, toluene, xylene),esters (ethyl, propyl, butyl, or hexyl acetate), halogenatedhydrocarbons (chlorobenzene), alkanols (methanol, ethanol, ethyleneglycol, ethylene glycol monomethyl ether) and ethers (diethyl ether,dibutyl ether) or mixtures thereof. A water-miscible or hydrophiliccosolvent may be added to the aqueous polymerization reactions in orderto ensure that the reaction mixture is present in the form of ahomogeneous phase during monomer conversion. Cosolvents which can beused in advantage with the present invention are chosen from thefollowing group, consisting of aliphatic alcohols, glycols, ethers,glycol ethers, pyrrolidines, N-alkylpyrrolidinones, N-alkylpyrrolidones,polyethylene glycols, polypropylene glycols, amides, carboxylic acidsand salts thereof, esters, organic sulfides, sulfoxides, sulfones,alcohol derivatives, hydroxy ether derivates, amino alcohols, ketonesand the like, and also their derivatives and mixtures.

The polymers prepared preferably have an average molecular weight of 50000 to 400 000 g/mol, more preferably between 100 000 and 300 000 g/mol.The average molecular weight is determined by size exclusionchromatography (SEC) or matrix-assisted laser desorption/ionization massspectrometry (MALDI-MS). Depending on reaction regime, the acrylic PSAsprepared by this process have a polydispersity of M_(w)/M_(n)<3.5.

For the use of the polyacrylates prepared by the inventive process aspressure sensitive adhesives, the polyacrylates are optimized byoptional blending with at least one resin. Tackifying resins to be addedinclude without exception all existing tackifier resins described in theliterature. Representatives that may be mentioned include pinene resins,indene resins and rosins, their disproportionated, hydrogenated,polymerized, esterified derivatives and salts, the aliphatic andaromatic hydrocarbon resins, terpene resins and terpene-phenolic resins,and also C5, C9 and other hydrocarbon resins. Any desired combinationsof these and other resins may be used in order to adjust the propertiesof the resulting adhesive in accordance with what is desired. In generalit is possible to use all resins which are compatible (soluble) with thecorresponding polyacrylate; reference may be made in particular to allaliphatic, aromatic, alkylaromatic hydrocarbon resins, hydrocarbonresins based on pure monomers, hydrogenated hydrocarbon resins,functional hydrocarbon resins, and natural resins. Explicit reference ismade to the depiction of the state of the art in the “Handbook ofPressure Sensitive Adhesive Technology” by Donatas Satas (van Nostrand,1989).

In a further advantageous development one or more plasticizers are addedto the PSA, such as low molecular weight polyacrylates, phthalates,whale oil plasticizers or plasticizer resins, for example.

The acrylic hotmelts may further be blended with one more additives suchas aging inhibitors, light stabilizers, ozone protectants, fatty acids,resins, nucleators, blowing agents, compounding agents and/oraccelerators.

They may further be admixed with one or more fillers such as fibers,carbon black, zinc oxide, titanium dioxide, solid or hollow glass(micro)beads, microbeads of other materials, silica, silicates andchalk, with the addition of blocking-free isocyanates being a furtherpossibility.

Particularly for use as a pressure sensitive adhesive it is an advantagefor the inventive process if the polyacrylate is applied preferably fromthe melt as a layer to a backing or to a backing material.

For this purpose the polyacrylates prepared as described above areconcentrated to give a polyacrylate composition whose solvent content is≦2% by weight. This process takes place preferably in a concentratingextruder. Then, in one advantageous variant of the process, thepolyacrylate composition is applied in the form of a layer, as a hotmeltcomposition, to a backing or to a backing material.

Backing materials used for the PSA, for adhesive tapes for example, arethe materials customary and familiar to the skilled worker, such asfilms (polyesters, PET, PE, PP, BOPP, PVC), nonwovens, foams, wovens andwoven films, and also release paper (glassine, HDPE, LDPE). This list isnot conclusive.

For the PSA utility it is particularly advantageous to crosslink thepolyacrylates following application to the backing or to the backingmaterial. For this purpose, in order to produce the PSA tapes, thepolymers described above are optionally blended with crosslinkers.Crosslinking may be brought about, advantageously, either thermally orby means of high-energy radiation; in the latter case, particularly bymeans of electron beams (EB) or, following the addition of suitablephotoinitiators, by means of ultraviolet radiation.

Preferred substances crosslinking under radiation in accordance with theinventive process are, for example, difunctional or polyfunctionalacrylates or difunctional or polyfunctional urethane acrylates,difunctional or polyfunctional isocyanates or difunctional orpolyfunctional epoxides. Further, it is also possible here to use anyother difunctional or polyfunctional compounds which are familiar to theskilled worker and are capable of crosslinking polyacrylates.

Suitable photoinitiators preferably include Norrish type I and type IIcleavers, some possible examples of both classes being benzophenone,acetophenone, benzil, benzoin, hydroxyalkylphenone, phenyl cyclohexylketone, anthraquinone, thioxanthone, triazine, or fluorenonederivatives, this list making no claim to completeness.

Also claimed is the use of the polyacrylate prepared by the inventiveprocess as a pressure sensitive adhesive.

Particularly advantageous is the use of the polyacrylate PSA prepared asdescribed for an adhesive tape, in which case the polyacrylate pressuresensitive adhesive may have been applied to one or both sides of abacking.

EXAMPLES

Test Methods

The following test methods were used in order to evaluate both theadhesive properties and general properties of the PSAs prepared.

180° Bond Strength Test (Test A)

A strip 20 mm wide of an acrylic PSA applied to polyesters as a layerwas applied in turn to steel plates. The PSA strip was pressed downtwice onto the substrate using a 2 kg weight. The adhesive tape was thenimmediately removed from the substrate at an angle of 180° and a speedof 300 mm/min. The steel plates were washed twice with acetone and oncewith isopropanol. The results are reported in N/cm and are averaged fromthree measurements. All measurements were carried out at roomtemperature.

Shear Strength (Test B)

A 13 mm wide strip of the adhesive tape was applied to a smooth steelsurface which had been cleaned three times with acetone and once withisopropanol. The area of application measured 20 mm*13 mm(length*width). The adhesive tape was then pressed onto the steelbacking four times using an applied pressure of 2 kg. At 80° C. a 1 kgweight, at room temperature a 1 kg or 2 kg weight, was fastened to theadhesive tape. The shear stability times measured are reported inminutes and correspond to the average of three measurements.

Gel Permeation Chromatography GPC (Test C)

The average molecular weight M_(w) and the polydispersity PD weredetermined by the company Polymer Standards Service, Mainz. The eluentused was THF containing 0.1% by volume trifluoroacetic acid. Measurementwas carried out at 25° C. The precolumn used was PSS-SDV, 5 μ, 10³ Å, ID8.0 mm×50 mm. Separation was carried out using the columns PSS-SDV, 5 μ,10³ and also 10⁵ and 10⁶ each with ID 8.0 mm×300 mm. The sampleconcentration was 4 g/l, the flow rate 1.0 ml per minute. Measurementwas carried out against PMMA standards.

Determination of the Gel Fraction (Test D)

The carefully dried, solvent-free adhesive samples are welded into apouch of polyethylene nonwoven (Tyvek web). From the difference in thesample weights before and after extraction with toluene the gel index isdetermined, i.e., the weight fraction of polymer that is not soluble intoluene.

Determination of the Conversion (Test E)

The conversion is determined gravimetrically and is reported as apercentage in relation to the amount by weight of the monomers used. Thepolymer is isolated by precipitation from methanol cooled to −78° C.,filtered off and then dried in a vacuum cabinet. The polymer is weighedand its weight is divided by the initial weight of the monomers used.The calculated figure corresponds to the percentage conversion.

Implementation of the Hotmelt Process in a Recording Extruder:

The shearing and thermal loading of the acrylic hotmelts was carried outusing the Rheomix 610p recording extruder from Haake. The drive unitavailable was the Rheocord RC 300p instrument. The instrument wascontrolled using the PolyLab System software. The extruder was chargedin each case with 52 g of pure acrylic PSA (˜80% fill level). Theexperiments were conducted at a kneading temperature of 140° C., arotary speed of 40 rpm and a kneading time of 5 hours. Thereafter thesamples, where possible, were dissolved again and the average molecularweight and the polydispersity of the material were determined via GPC.

Preparation of the nitroxide Ia (2,2,5-trimethyl-4-phenyl-3-azahexane3-nitroxide):

The procedure adopted was analogous to the experimental instructionsfrom Journal of American Chemical Society, 121, 16, 3904-3920, 1999.

Preparation of the alkoxyamine IIa(2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane):

procedure adopted was analogous to the experimental instructions fromJournal of American Chemical Society, 121, 16, 3904-3920, 1999.

General Implementation of the Nitroxide-Controlled Polymerizations:

A mixture of the alkoxyamine IIa, the nitroxide Ia (5 mol % based onalkoxyamine IIa), and 2.5 mol % of Vazo 88™ (2.5 mol % based onalkoxyamine IIa) are mixed with the monomer (85% strength solution inxylene), and the mixture is degassed a number of times and then heatedat 125° C. under an argon atmosphere. The reaction time is 24 h.Determination of molecular weight and polydispersity were carried outvia GPC.

Production of the Reference Specimens

Example 1

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 28 g of acrylic acid, 292 g of 2-ethylhexyl acrylate, 40 gof methyl acrylate and 300 g of acetone/isopropanol (93:7). Nitrogen gaswas passed through the reaction with stirring for 45 minutes, afterwhich the reactor was heated to 580° C. and 0.2 g of azoisobutyronitrile(AIBN, Vazo 64™, DuPont) was added. Then the external heating bath washeated to 750° C. and the reaction was carried out constantly at thisexternal temperature. After a reaction time of 1 hour a further 0.2 g ofAIBN was added. After 3 hours and 6 hours, in each case 150 g ofacetone/isopropanol (93:7) mixture were added for dilution. In order toreduce the remaining initiators, in each case 0.4 g ofbis(4-tert-butylcyclohexanyl) peroxydicarbonate (Perkadox 16™, AkzoNobel) was added after 8 hours and after 10 hours. After a period of 22hours the reaction was terminated and the product cooled to roomtemperature.

The average molecular weight and the polydispersity were determined bymeans of test C.

In order to investigate the thermal aging, the adhesive was freed fromthe solvent in a vacuum drying cabinet and then subjected to shearingand thermal loading in the recording extruder in accordance with themethod described above.

In order to examine the technical adhesive properties, the driedpolyacrylate was applied to a 23 μm PET backing provided with Saranprimer, application of the polyacrylate taking place at a rate of 50g/m² using a laboratory roll coater, and the applied polyacrylate wasthen irradiated with 40 kGy at an acceleration voltage of 230 KV, usingan EB unit from Crosslinking, and cured. For technical adhesiveassessment, test methods A and B were conducted.

Example 2

The procedure of example 1 was repeated. The polymerization was carriedout using 28 g of acrylic acid, 20 g of methyl acrylate, 20 g of styreneand 332 g of 2-ethylhexyl acrylate. The initial monomer concentrationwas raised to 80%.

Nitroxide-Controlled Polymerizations

Example 3

28 g of acrylic acid, 292 g of 2-ethylhexyl acrylate and 40 g of methylacrylate were used. As initiators and regulators, 325 mg of alkoxyamine(IIa), 11 mg of nitroxide (Ia) and 12 mg of Vazo 88™ (DuPont) wereadmixed. The polymerization was conducted in accordance with the generalimplementation instructions for nitroxide-controlled polymerizations.For workup and further processing the procedure of Example 1 wasadopted.

To determine the conversion, the polymerization was repeated and thenthe procedure of test method E performed.

Example 3′

28 g of acrylic acid, 292 g of 2-ethylhexyl acrylate and 40 g of methylacrylate were used. As initiators and regulators, 325 mg of alkoxyamine(IIa) and 11 mg of nitroxide (Ia) were used. The monomers and thenitroxides are mixed in xylene (85% strength solution in xylene), andthe solution is degassed a number of times and then heated at 125° C.under an argon atmosphere. The reaction time is 24 h. Thereafter theconversion was determined by test method E.

Example 4

28 g of acrylic acid, 20 g of methyl acrylate, 20 g of styrene and 332 gof 2-ethylhexyl acrylate were used. As initiators and regulators, 325 mgof alkoxyamine (IIa), 11 mg of nitroxide (Ia) and 12 mg of Vazo 88™(DuPont) were admixed. The polymerization was conducted in accordancewith the general implementation instructions for nitroxide-controlledpolymerizations. For workup and further processing the procedure ofExample 2 was adopted.

To determine the conversion, the polymerization was repeated and thenthe procedure of test method E performed.

Example 4′

28 g of acrylic acid, 20 g of methyl acrylate, 20 g of styrene and 332 gof 2-ethylhexyl acrylate were used. As initiators and regulators, 325 mgof alkoxyamine (IIa) and 11 mg of nitroxide (Ia) were used. The monomersand the nitroxides are mixed in xylene (85% strength solution inxylene), and the solution is degassed a number of times and then heatedat 125° C. under an argon atmosphere. The reaction time is 24 h.Thereafter the conversion was determined by test method E.

Example 5

40 g of acrylic acid and 360 g of 2-ethylhexyl acrylate were used. Asinitiators and regulators, 325 mg of alkoxyamine (IIa), 11 mg ofnitroxide (Ia) and 12 mg of Vazo 88™ (DuPont) were admixed. Thepolymerization was conducted in accordance with the generalimplementation instructions for nitroxide-controlled polymerizations.For workup and further processing the procedure of Example 3 wasadopted.

Example 6

12 g of acrylic acid, 194 g of 2-ethylhexyl acrylate and 194 g ofn-butyl acrylate were used. As initiators and regulators, 325 mg ofalkoxyamine (IIa), 11 mg of nitroxide (Ia) and 12 mg of Vazo 88™(DuPont) were admixed. The polymerization was conducted in accordancewith the general implementation instructions for nitroxide-controlledpolymerizations. For workup and further processing the procedure ofExample 3 was adopted.

Example 7

8 g of acrylic acid, 4 g of methyl acrylate, 40 g ofN-tert-butylacrylamide and 348 g of 2-ethylhexyl acrylate were used. Asinitiators and regulators, 325 mg of alkoxyamine (IIa), 11 mg ofnitroxide (Ia) and 12 mg of Vazo 88™ (DuPont) were admixed. Thepolymerization was conducted in accordance with the generalimplementation instructions for nitroxide-controlled polymerizations.For workup and further processing the procedure of Example 3 wasadopted.

Results

The comparison of examples 1 and 2 with 3 and 4 demonstrates theadvantages of polyacrylate pressure sensitive adhesives prepared bynitroxide-controlled polymerization. The reference specimens (examples 1and 2) were prepared conventionally in a free radical polymerization.For comparison, the polyacrylates in examples 3 and 4, with theidentical comonomer composition, were prepared by nitroxide-controlledpolymerization. The results of the polymerizations are illustrated intable 1:

TABLE 1 M_(W) Polydispersity Example [g/mol] PD 1 489 500 5.9 2 532 0006.3 3 378 000 2.8 4 393 000 2.9

As a result of the free radical polymerization, examples 1 and 2 exhibita high polydispersity. Isopropanol as regulator reduces the averagemolecular weight but generally broadens the molecular weightdistribution. As a result of the nitroxide-controlled polymerization,significantly lower polydispersities are obtained. Moreover, there is adistinct improvement in the hotmelt processing properties. For thispurpose examples 1 to 4 were subjected to thermal loading and shearingin a hotmelt kneading apparatus at 140° C. for several hours. Thereafterthe gel index was measured, in order to investigate the effect of thedamage on the polymer. The results are illustrated in table 2:

TABLE 2 Example Gel index [%] 1 11 2 8 3 0 4 0

Examples 1 and 2 show distinct aging after shearing load. Thecomposition possesses a gel index of 8% (example 2) or 11% (example 1).Partially gelled polyacrylates cannot be applied either in the hotmeltprocess or from solution as PSAs. Consequently, aged PSAs of this kindare completely unsuitable for practical application. Contrastingly,examples 3 and 4 show no aging phenomena, such as gelling, for example.As a result of the nitroxide-controlled polymerization, the polymerscontain nitroxides as end groups, which at high temperatures are able toact as radical scavengers in situ. As a result of the polymerizationprocess, therefore, an aging inhibitor is incorporated directly into thePSA. The polyacrylates prepared by this route can be readily processedby the hotmelt process and, accordingly, can be used preferentially asPSAs.

In order to assess the technical adhesive properties the PSAs arecompared with one another in table 3:

TABLE 3 SST (RT, BS-steel Example 10 N) [N/cm] 1     2475 3.8 2     34903.7 3 +10 000 3.6 4 +10 000 3.4 SST: Shear stability times RT: Roomtemperature BS: Bond strength

The narrower distribution of the molecular weights brings about a moreefficient network in the case of EB crosslinking. The shear strength ofthe PSAs is raised. For an identical comonomer composition, examples 3and 4 exhibit a much higher shear strength as compared with examples 1and 2. The effect as far as the bond strengths are concerned isnegligible.

In order to examine the efficiency of the preparation process of theinvention, the conversion rate of examples 3 and 4 was measured. Inparallel thereto, conventional nitroxide-controlled polymerizations wereconducted which contained no additions of Vazo 88™ (DuPont) and thus donot represent an additional source of radicals which might acceleratethe polymerization. The results from these comparative investigationsare listed in table 4.

TABLE 4 Conversion Example [%] 3  95 4  94 3′ 83 4′ 78

The conversion measurements demonstrate that the addition of Vazo 88™considerably increases the polymerizations and that after a reactiontime of just 24 h it is possible to achieve a conversion of well above90%. The polymerizations conducted for comparison, without free additionof initiator (examples 3′ and 4′), lie well below 90% and are thereforenot very suitable for preparing acrylic PSAS, since residualmonomers—even in a hotmelt concentration process—are very difficult toremove and these fractions present in the PSA tape product may giverise, for example, to skin irritations.

In order to examine the process of the invention for producing acrylicPSA tapes, further acrylic PSAs with different comonomer compositionswere prepared by means of nitroxide-controlled polymerization. Theresults of the polyacrylates applied from the melt are illustrated intable 5.

TABLE 5 SST (RT, BS-steel Example 10 N) [N/cm] 5 +10 000     4.0 6 61954.8 7 3680 5.0 SST: Shear stability times RT: Room temperature BS: Bondstrength

Examples 5 to 7 demonstrate that other comonomers as well can be used.Thus it is also possible to prepare relatively soft acrylic PSAs whichpossess a higher bond strength on steel, for example. The shear strengthof the acrylic hotmelt PSA described is also very high.

1. An initiator system for free-radical polymerizations, comprised of acombination of compounds of the formulae

in which R′, R″, R′″, R″″ are chosen independently of one another andrepresent a) branched and unbranched C₁ to C₁₈ alkyl radicals; C₃ to C₁₈alkenyl radicals; C₃ to C₁₈ alkynyl radicals; b) C₃ to C₁₈ alkynylradicals; C₃ to C₁₈ alkenyl radicals; C₁ to C₁₈ alkyl radicalssubstituted by at least one OH group or halogen atom or a silyl ether;c) C₂-C₁₈ hetero alkyl radicals having at least one oxygen atom and/oran NR group in the carbon chain; R being chosen from one of groups a),b), and d) to g), d) C₃-C₁₈ alkynyl radicals, C₃-C₁₈ alkenyl radicals,C₁-C₁₈ alkyl radicals substituted by at least one ester group, aminegroup, carbonate group and/or epoxide group and/or by sulfur and/or bysulfur compounds, especially thioethers or dithio compounds; e) C₃-C₁₂cycloalkyl radicals; f) C₆-C₁₀ aryl radicals; g) hydrogen; X representsa group with at least one carbon atom wherein the free radical X•derived from X has the ability to initiate a polymerization ofethylenically unsaturated monomers and further comprising thermallydecomposing radical-forming azo and/or peroxo initiators, wherein thecompounds of formula (I) are present in a proportion of from 1 mol. % upto 10 mol. %, based on amount of the compounds of the formula (II),and/or the azo and/or peroxo initiators are present in a proportion from1 to 10 mol. %, based on the amount of compound of the formula (II). 2.The initiator system of claim 1, wherein compound (I) is the compound offormula (Ia) and compound (II) is the compound of formula (IIa):


3. A process for preparing acrylic pressure sensitive adhesives, whereina monomer mixture having at least 70% by weight of ethylenicallyunsaturated compounds is subjected to free-radical polymerizationinitiated by the initiator system of claim
 1. 4. The process of claim 3,wherein said ethylenically unsaturated compounds are monomers of theformula

in which R₁=H or CH₃ and R₂=H or is an alkyl chain having 1-20 carbonatoms, or vinyl compounds are used as additional monomers, with afraction of up to 30% by weight, or both.
 5. The process of claim 3,wherein resin or other additives, selected from the group consisting ofaging inhibitors, light stabilizers, ozone protectants, fatty acids,plasticizers, nucleators, blowing agents, accelerators fillers endcombinations thereof are added to the monomer mixture or to the acrylicpressure sensitive adhesive.
 6. The process of claim 3, whereincrosslinkers have been added to the polyacrylate composition to becrosslinked.
 7. The process of claim 3, wherein the pressure sensitiveadhesive is applied to a backing as a melt.
 8. The initiation system ofclaim 1, wherein said amount of compounds of formula (I) is 3 mol. % to7 mol. %, and said amount of azo and/or peroxo compounds is from 3 to 7mol. %.
 9. The process of claim 4, wherein said vinyl compounds areselected from the group consisting of vinyl esters, vinyl halides,vinylidene halides, nitrites of ethylenically unsaturated hydrocarbons,a combinations thereof.
 10. The process of claim 6, wherein saidcrosslinkers are selected from the group consisting of difunctionalacrylates, polyfunctional acrylates, difunctional methacrylates,polyfunctional acrylates, photoiniators and combinations thereof.